Ecology of small neritic fishes in the western Gulf of Alaska: II. Consumption of krill in relation to krill standing stock and the physical environment

Krill (Euphausiacea) is a patchily distributed taxon whose availability may limit neritic fishes in temperate oceans. In the western Gulf of Alaska, krill–fish aggregations were associated with high-flow areas over the shelf. We examined fish impacts on krill standing stocks in areas of different temperature, salinity, and net current velocity. Samples were collected during September 2000, 2001, and 2003 over a 48-site grid within a known walleye pollock nursery. Krill were a dietary staple of the dominant fishes: walleye pollockTheragra chalcogramma, capelinMallotus villosus, and eulachonThaleichthys pacificus, but their proportion in diets varied by predator species and predator length. Predators daily consumed ≤12% of standing stocks; thus, krill appeared to be amply available. However, the krill consumed by eulachon and large (>120 mm) walleye pollock were, on average, large compared to krill in plankton samples; therefore, standing stock sizes might have been overestimated by including small krill. A compensatory response in consumption occurred during 2001 in proximity to the Shelikof sea valley due to increased per capita predation rates and local concentration of Age-1+ walleye pollock and eulachon. High abundance of krill in 2001 was associated with high ocean current flow. No compensatory response was observed where local standing stocks were dominated by small krill. Thus, apparent bottom-up influences of ocean currents on krill abundance in neritic areas can be partly compensated by localized top-down predation from nektonic fishes having prey size preferences that match available prey sizes.